This invention relates to a method and apparatus for monitoring optical power in waveguiding structures such as optical fibers and, in particular, to monitoring such power levels by the heat they generate.
Optical waveguides are important for a wide variety of applications including the transmission of optical signals for optical communication systems and optical power for pumping lasers and optical amplifiers.
An optical waveguide typically comprises an inner core region of transparent material having a first index of refraction and a peripherally surrounding cladding region of material having a lower index of refraction. A light beam entering the core within the waveguide acceptance angle is guided along the core by total internal reflection from the core/cladding interface. The waveguide is typically optical fiber.
Monitoring the level of optical power in a waveguide is important in a number of practical applications. For example, in optical communication systems, the amount of amplification provided to a transmitted signal depends in part on the level of optical pump power provided to a rare-earth doped waveguide amplifier. The optical pump power is typically provided by a semiconductor diode and transmitted from the diode to the amplifier by a length of waveguide.
Methods of monitoring the diode directly are not reliable. Methods based on measuring the power to or from the diode theoretically can measure the pump light generated but do not provide an accurate measure of the generated light actually launched into the transmitting waveguide.
Alternative methods based on coupling pump light out of the transmitting waveguide are also disfavored. First, they produce undesirable optical loss. Second, they are sensitive to fluctuations in the polarization of light in the waveguide. Accordingly there is a need for an improved method of monitoring optical power in a waveguide.
In accordance with the invention, the optical power level in an optical waveguide is monitored by enclosing a length of the waveguide within an insulated cavity of comparable length and cross section, measuring a first temperature T1 within the cavity, measuring a second temperature T2 outside the cavity and deriving from the difference, T1xe2x88x92T2, a measure of the optical power level. Exemplary apparatus for monitoring the optical power level in an optical waveguide comprises a substrate with an insulated groove for receiving an optical fiber, an insulated lid for sealing the fiber within the groove, and internal and external temperature sensors.